Flaky test check_runtimes: [02:47:43]RUNTIME: Cannot read null.assigned_equipment_preset - code/game/objects/items/devices/radio/headset.dm@372

Flaky tests were detected in [this test run](https://github.com/cmss13-devs/cmss13/actions/runs/27998325222/attempts/1). This means that there was a failure that was cleared when the tests were simply restarted.
Failures:
```
check_runtimes: [02:47:43]RUNTIME: Cannot read null.assigned_equipment_preset - code/game/objects/items/devices/radio/headset.dm@372 at code/defines/../modules/unit_tests/check_runtimes.dm:10
```

Start with the failing check_runtimes test at code/defines/../modules/unit_tests/check_runtimes.dm:10 and inspect code/game/objects/items/devices/radio/headset.dm:372. Reproduce the test failure and trace when assigned_equipment_preset is read from null; done means the test no longer reports this runtime failure across repeated runs.

Written by the indexing model from the issue text.